Significant amounts of precious metal, especially gold, are found dispersed in pyritic sulfide minerals, such as for example in pyrite, marcasite, pyrrhotite, arsenopyrite and/or arsenian pyrite. Often, very little of the gold contained in these sulfide minerals is recoverable by direct leaching with cyanide or other gold lixiviants. Rather, it is typically necessary to decompose the sulfide minerals to a significant extent to expose the gold and make the gold available to be leached. Sulfide mineral decomposition may involve an oxidative treatment, for example pressure oxidation or biooxidation. Processing may include preparing a sulfide concentrate by flotation prior to the oxidative treatment. Preparing a concentrate reduces the amount of material that must be processed in the oxidative treatment and provides higher sulfide sulfur concentrations that may be beneficial to drive desired reactions during some oxidative treatment techniques. For example, one oxidative treatment is acidic pressure oxidation in which a sulfide-containing mineral material, such as an ore, concentrate or ore/concentrate blend, is contacted with oxygen in an autoclave at elevated temperature and pressure in an acidic environment. In the autoclave, oxygen gas reacts with sulfide sulfur resulting in decomposition of sulfide minerals and generation of sulfuric acid. The sulfide sulfur oxidation is exothermic and the process may be thermally autogenous provided the feed contains a sufficiently high concentration of sulfide sulfur to generate adequate heat and sufficiently high acid concentrations.
Some ores contain significant quantities of acid-consuming carbonate that complicates processing. Many carbonate minerals, for example calcite, magnesite, siderite and dolomite, will react with sulfuric acid resulting in decomposition of the carbonate, generation of carbon dioxide and formation of sulfate salts. A small amount of acid-consuming carbonate in mineral material feed to acidic pressure oxidation feed may be acceptable, but as carbonate concentrations become larger consumption of sulfuric acid by the carbonate can be a significant detriment to the pressure oxidation operation. One way to address high concentrations of carbonate material in sulfide ores to be pressure oxidized is to pre-treat the ore with sulfuric acid to decompose the carbonates prior to being fed to the pressure oxidation autoclave, so that the carbonate will not be available to react in the autoclave and therefore will not interfere with desired reactions during pressure oxidation. However, the cost of sulfuric acid consumed in such a pre-treatment operation can be significant. For ores with high gold concentrations, the cost of pre-treatment may be justifiable, but for many carbonate-containing ores such pre-treatment may be cost prohibitive even with carbonate levels that are in the range of one weight percent, or even less in some cases.
Carbonate content in an ore can also be a significant problem for flotation processing to prepare a sulfide concentrate for oxidative treatment. For many sulfide gold ores, flotation to prepare a sulfide concentrate may be most effective if performed at an acidic pH, and feed slurries to flotation are adjusted to the desired acidic pH by addition of acid, commonly sulfuric acid. However, when the ore contains significant acid-consuming carbonate, significant acid is consumed to decompose the carbonate before the pH of the ore slurry can effectively be adjusted to the desired acidic pH for flotation. Additionally, as a result of the acidification precipitates may form that may have a very small size that can complicate flotation and post-flotation filtration of flotation concentrates, such as by causing filter plugging. Especially when mineral material contains significant acid-consuming carbonate in the form of calcite, very fine particles of calcium sulfate (gypsum) precipitate may form. The presence of such precipitates during flotation may interfere with flotation performance, which may lead to a need to perform flotation at a lower slurry solids density. Also, the presence of such precipitates in the flotation concentrate may interfere with effective dewatering of flotation concentrates, such as by filtration. Filtration plugging can become a significant problem as a result of such very fine precipitates.